1. Field of Invention
This invention relates generally to three dimensional coordinate measuring machines (CMM's). More specifically this invention relates to an arm and counterbalance mechanism for use on a CMM which provides increased reliability and adjustability.
2. Description of the Prior Art
It is well known in the art to utilize a CMM to measure objects in a space in terms of their X, Y, and Z coordinates commonly referring to length, width and height, respectively. Advancement in the art has led to lightweight portable CMM's well suited for general industrial applications. Such a CMM is disclosed in U.S. Pat. No. 5,402,582 which is commonly assigned to the assignee hereof and incorporated herein by reference.
One of the above mentioned advancements in the art of portable CMM's is a light weight multi-jointed manually positionable measuring arm, shown generally in FIG. 1 at 10. Measuring arm 10 is comprised of a plurality of transfer housings 12 (with each transfer housing comprising a joint and defining one degree of rotational freedom) and extension members 14 attached to each other with adjacent transfer housings being disposed at right angles to define a movable arm 10 preferably having multiple degrees of freedom. At one end of arm 10 is attached a base 20. At the end of arm 10 opposite base 20 is attached a probe 15.
Referring to FIG. 2, the measuring arm 10 of the prior art further comprises a torsional spring 16 positioned in a joint 22 near base 20 of measuring arm 10. The torsional spring 16 provides a counter balance force to offset the weight of the arm and ease manipulation thereof by an operator. An air piston shock absorber 18 is mounted on base 20 of arm 10 in intimate contact with joint 22 such that piston 18 is fully compressed when arm 10 is in rest position, as is shown. Piston 18 is fully decompressed and awaiting retraction of arm 10 when said arm is fully extended. Air piston 18 absorbs the shock load accompanying the spring coiled retraction of arm 10 by exerting a force opposite to said retraction.
The base 20 of CMM arm 10 of the prior art is typically mounted in the horizontal plane. Referring again to FIG. 2, the recoiled torsional spring 16 generates a compensating torque at the base 20 of the arm 10 in a direction 24 to considerably reduce the weight of the arm 10, said weight acting in a direction 26 when arm 10 is extended. Such alignment allows for a counterbalanced use of the arm 10 when base 20 is mounted in the horizontal plane as described herein above. However, there are many applications of CMM's where it is advantageous to mount the arm perpendicular to or inverted to the above discussed original mounted horizontal plane. For instance, it is often desired in the art to mount the arm 10 to a wall or to a ceiling to facilitate a particular use of the CMM. This mounting naturally changes the direction 26 of the weight of the arm 10 relative to said arm. The compensating torque 24, however, created by torsional spring 16 remains the same. Thus, the effect of the arm's spring coiled counterbalancing mechanism is diminished. Without the aid of the counterbalancing mechanism, use of the arm 10 may be cumbersome.
Prior art CMM arms, as discussed above, do not readily allow multiple applications requiring changability of a single CMM. For instance, a single CMM may be used by a variety of operators who may require different counterbalancing forces to effect a proper movement of the machine. Different end probes may be required for various application and alternative mountings may be necessary. The CMM arms of the prior art do not readily allow adjustability of the counterbalancing mechanism to compensate for the change in forces acting upon the arm associated with use of various mountings and end probes.
The positioning of the torsional spring counterbalance of the prior art CMM arm causes a high overhung load. As discussed herein above the counterbalance mechanism is positioned in a joint near the base of the arm. Such positioning creates a substantial moment arm from the neutral axis of joint previous to the mechanism. The majority of the weight of the measuring arm acts on this moment arm and creates a considerable load on the joint and on the base thereby reducing operability and increasing stress on the base assembly of the CMM arm.
Thus the need has arisen for a CMM arm with a mechanism which allows for the counterbalanced use of the arm in a variety of mountings, with a variety of end probes which prevents overhang stress on the base of the arm and provides ease in changability.